Electric motors are used in a variety of electrical equipment. For example, wafer stages also utilize linear or planar electric motors to position a wafer during photolithography and other semiconductor processing.
A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. An armature is disposed in a space defined between the pairs of opposing magnets. The space extends along a direction along which the armature may move. The armature has windings of a conductor which, upon application of an electrical current, the electric current interacts with the magnetic fields of the magnet pairs to exert force on the armature, causing the armature to move. The armature moves along the length of the space. When the armature is attached to a wafer stage, the wafer stage experiences the same force as and moves in concert with the armature.
Two-dimensional planar electric motors are utilized in semiconductor manufacturing. For example, U.S. Pat. No. 4,654,571, entitled "Single Plane Orthogonally Moveable Drive System," to Hinds and U.S. Pat. No. 4,535,278, entitled "Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus," to Asakawa discuss two-dimensional electric motors and are incorporated herein by reference in their entireties. The two-dimensional motors have two-dimensional arrays of magnets and an armature or an array of coils movable relative to each other in two or more directions rather than magnet tracks having a space along which an armature moves along a single direction. The magnet array and the armature may move relative to each other in more than two dimensions depending upon the design. Conventional two-dimensional motors typically have an array of magnets and an armature having one or more coils disposed on one side of the array of magnets.
U.S. Pat. No. 5,623,853, entitled "Precision Motion Stage with Single Guide Beam and Follower Stage," to Novak et al. and U.S. Pat. No. 5,528,118, entitled "Guideless Stage With Isolated Reaction Stage," to Lee discuss examples of semiconductor fabrication equipment and are incorporated herein by reference in their entireties.
A platform may be attached to a two-dimensional motor and the platform can be moved in two or more dimensions by the motor. For example, a wafer stage in semiconductor processing equipment may be attached to an armature or a magnet array of a two-dimensional electric motor and the two-dimensional motor would control positioning of the wafer stage.
One problem with conventional magnet arrays is their relatively low magnetic flux to mass ratio. Without a backing by a magnetically permeable material, the magnetic flux is relatively low. A magnetically permeable backing facilitates the completion of magnetic flux paths between magnets having opposite polarities. The magnetic flux of the magnet array is greater when the flux paths are completed by the magnetically permeable backing. However, magnetically permeable backing, such as iron, are relatively heavy and decreases the magnetic flux to mass ratio of the magnet array. If the flux paths between the magnets of opposite polarities could still be completed without the use of a heavy magnetically permeable backing, the magnetic flux to mass ratio would be improved. For electric motors having a moving magnet array and a stationary coil array rather than a moving coil array and a stationary magnet array, this reduction in mass would improve the efficiency of the electric motor.
There is, therefore, a need for a compact motor which provides high speed and precision and energy efficient operation.